The present invention is directed to methods of preparing tunable and removable antireflective coatings based on amorphous carbon films.
As feature sizes shrink below 0.35 xcexcm for logic and memory chips, critical dimension (CD) tolerances become more stringent. The variations in CD are directly related to substrate reflectance, which sharply increases at DUV (deep ultraviolet) wavelengths (365, 248, 193 nm). Substrate reflectance is notorious for producing standing wave effects and notching of the photoactive resist. Standing waves are thin film interference (TFI) or periodic variations of light intensity through the resist thickness. These light intensity variations are introduced because planarization of the resist presents differing thickness through the underlying topography. Notching results from substrate topography and nonuniform substrate reflectance which causes local variations in exposure energy on the resist. To effectively dampen this reflectance, xe2x80x9cspin-onxe2x80x9d ARC and dry deposited antireflective coatings (ARC) are used.
The antireflective coating formed at the substrate/resist interface is called bottom ARC (BARC). Spin-on BARCs are widely used in the manufacturing of computer chips, but they do present severe limitations. For example, they have poor optical tunability, which means that their index of refraction n and their extinction coefficient k can not be finely tuned to match resist and substrate optical properties. Furthermore, with feature dimensions shrinking below one quarter micron, these films cannot be conformally deposited over topography. A review of presently used spin-on BARC can be found in xe2x80x9cResist Enhancement with Antireflective Coatingsxe2x80x9d, Semiconductor International, July 1996, pages 169-175, by Ruth deJule.
To overcome these problems, vapor deposited BARCs are being developed. These films are deposited preferably by plasma enhanced chemical vapor deposition. Thus, they can be conformally deposited over topography. Also their optical performance is far superior to that of spin-on BARCs. Applied Material Co. proposed a vapor deposition silicon oxynitride as a DUV BARC as reported in Solid State Technology, page 62, July 1996, in an article titled xe2x80x9cApplied Developed ARC Using Silane-based CVDxe2x80x9d. These films do have good optical tunability, i.e. n and k can be varied by changing process conditions, but their removal after DUV exposure can be difficult. The fluorine chemistry used to remove the SiN film also etches the structures underneath.
F. D. Bailey et al. (U.S. Pat. No. 5,569,501), proposed to use an amorphous hydrogenated carbon film (a-C:H) deposited by vapor deposition from a hydrocarbon helium plasma as a BARC. These films were found to give lithographic performance superior to that of spin-on ARCs and they could be removed easily in an oxygen plasma without damaging the structures underneath. However, the index of refraction n could not be tuned over a wide range of values by changing process conditions.
It is an object of the invention to provide by vapor deposition an amorphous carbon film which has the required optical properties to form a DUV (365, 248, 193 nm) BARC. The method can be extended easily to manufacturing tools currently used in the semiconductor industry.
It is another object of the invention to carry out the depositions by vapor deposition in an argon/hydrocarbon/helium/hydrogen/fluorocarbon/nitrogen/oxygen mixture, preferably with the hydrocarbon being cyclohexane or acetylene and the fluorocarbon preferably being hexafluorobenzene. Higher indexes of refraction are obtained by limiting or excluding fluorocarbon (HFB) flow in the plasma chamber, while lower indexes of refraction are obtained by increasing the HFB flows and limiting or excluding the hydrocarbon gas flow. Finer optical tunability can be achieved by using proper flow amounts of nitrogen and/or oxygen. Hydrogen is used to tune optical properties as well as to improve film durability.
It is another object of the invention to provide a method of depositing a film structure by vapor deposition to be used as a BARC with optical properties optimized to minimize reflections at the resist/substrate interface. The process gas chemistry and the process parameters in general are uniquely optimized to achieve the preselected optical properties.
It is another object of the invention to provide a method of depositing a film structure by vapor deposition with optical properties that can be fine-tuned by continuously changing the refractive index n from bottom to top of the ARC layers to build a gradient effect. More importantly, if the n of the ARC layers is perfectly matched to the adjacent layers, there will be no reflection at the resist/ARC interface, greatly improving CD control.
A broad aspect of the present invention is the vapor deposition of an a-C:X:H film having tunable optical properties, where X is fluorine, nitrogen, oxygen, or silicon, or combinations thereof.
A more specific aspect of the method according to the present invention is the deposition of a hydrogenated carbon film by vapor deposition from a hydrocarbon/fluorocarbon/hydrogen plasma, optionally also maintaining a low amount of nitrogen and/or oxygen, such as about 1 standard ccm per deposition. The films produced herein can be tuned to match the substrate optical properties at 365, 248, 193 nm, making them extremely useful for bottom antireflective coatings. Additionally, unlike films being explored by others, the films formed in the present invention can be deposited conformally on topography and also can be readily removed in an oxygen and/or fluorine reactive ion etching process, thereby facilitating patterning for chip fabrication.
Another more specific aspect of the present invention is the process of depositing an amorphous carbon film by vapor deposition which comprises the steps of: admixing of hydrocarbon, fluorocarbon and hydrogen gases, optionally with a low amount of oxygen and/or nitrogen gases; providing a reactor chamber containing a cathode and the substrate; introducing the above gas mixture into the chamber; and applying an rf bias potential to the cathode to initiate a plasma and to deposit the a-C:X:H film on the substrate by vapor deposition.
More specifically, the present invention provides a method of depositing an amorphous carbon film by using a gas mixture which comprises hexafluorobenzene, hydrogen, cyclohexane and acetylene, which may or may not be diluted by helium and/or argon gases to reactively deposit a film by vapor deposition. By employing this method, the index of refraction n and the extinction coefficient k can be independently optically tuned at UV and DUV wavelengths. More specifically, the UV and DUV index of refraction n and extinction coefficient k can be tuned from about 1.40 to about 2.1 and about 0.1 to about 0.6, respectively, at 365, 248, and 193 nm. Therefore, these films meet all the requirements needed to be used as bottom antireflective coatings, since they now match the optical properties of both the substrate and the photoresist.